Interesting comments here. Our old O'Brien knee board just has a strap, no hook so we never thought or knew to remove it. As they do more tricks it's nice to have. Ours do spins, wake launches/jumps, etc. so they like the strap tight.
I've taught and towed kids (mostly teens+) on knee boards a lot over they years because it always seems the least intimidating to them behind the boat. It does beat you to death, very hard on the knees. Wake boards are MUCH gentler and easy to get up on IMHO. If you're porpoising a knee board, it's way too fast. When I teach tow drivers, I tell them just to "squeeze evenly on the throttle" to plane the board gently, not jam it, allowing you to shinny up onto your knees on the board, raising the grab on the rope, stabilize the board, then cinch the strap up. Once level, the driver should find a good cruise speed for the size of board and weight of rider.
#1 rule in all board/ski sports (water and snow), is DON'T look down because that's the direction you go - face plant! And it hurts on a knee board. Keep head up, eyes forward to where you want to go, back straight, hips level/forward. Flexing (not locked) arms towards hips gives control and a good pivot point for turning, cutting, and such. Have fun!